Alterations in the permeability of the alveolar membranes will be measured in patients with acute or chronic interstitial lung disease. The clearance of 99mTc-DTPA from the lungs following administration in an aerosol is used for this purpose. Among patients to be studied are those with respiratory distress syndrome, congestive failure and pneumonoconioses. Measurements will be made of the permeability of the alveolar-capillary membranes to 14CO2 following instillation of NaH14CO3 into bronchi of perfused rabbit lungs. An effort will be made to quantitate the number of receptor sites for both alpha and beta adrenergic agents accessible to plasma in this preparation as well as angiotensin converting enzyme on the pulmonary endothelial surface. A study will be made of the effect of osmolality upon passage of solutes across the alveolar capillary barrier.